1. Field of the Invention
The present invention relates to a method of heat-treating a silicon nitride sintered body in order to improve its crystallinity and thereby improving its high-temperature strength. More particularly the present invention relates to a method of heat-treating a silicon nitride sintered body by which the desired high-temperature strength can be achieved in a short period of time through improving its crystallinity.
2. Description of the Related Art
Silicon nitride sintered bodies are expected to be used as structural ceramics for gas turbine parts or other embodiments that require operability under severe conditions at high temperatures, etc. because of their unique properties such as high strength, heat resistance, thermal shock resistance, wear resistance, oxidation resistance, etc. However, since the silicon nitride itself is a material which cannot be easily sintered, various sintering aids are added.
These sintering aids function to form a glassy boundary phase in the boundaries of silicon nitride grains, thereby accelerating the sintering of silicon nitride. In the course of sintering, it is presumed that a small amount of SiO.sub.2 present in the surface of silicon nitride particles contributes to the formation of the glass phase in the boundaries of silicon nitride grains. By using these sintering aids, a sintering reaction easily proceeds. However, a glass phase of a low melting point forms in the boundaries of silicon nitride grains in the resulting sintered bodies, thereby deteriorating the high-temperature strength of the sintered body.
Accordingly, the following attempts were proposed to prevent a decrease in the strength of the silicon nitride sintered bodies at a high temperature. For example, there are methods of selecting particular types and amounts of sintering aids, thereby achieving the accelerated crystallization of boundary phases in silicon nitride grains: a method of adding oxides; of rare earth elements such as yttrium oxide, a method of adding both yttrium oxide and other oxides such as aluminum oxide (for instance, Japanese Patent Publication No. 49-21091); and a method of adjusting the formulation of yttrium oxide, aluminum oxide and silicon nitride to improve oxidation resistance at 1400.degree. C. (Japanese Patent Laid-Open No. 63-185863).
However, the resulting silicon nitride sintered bodies obtained by these methods show insufficient high-temperature strength, and thus fail to meet the requirements for gas turbine parts, etc.
In addition, there is another method for improving the high-temperature strength of the silicon nitride sintered bodies, according to this method the silicon nitride sintered bodies are embedded an aluminum nitride powder and heated, thereby decomposing the oxides (SiO.sub.2) present in the glass boundary phase of the sintered body. The thus formed to SiO which is vaporized, which results in a decrease in the oxygen content (the content of SiO.sub.2)in the glass boundary phase.
However, according to this method of lowering the content of SiO.sub.2, the heat treatment should be conducted for a long period of time, e.g., about 100 hours, in order to effectively increase the strength of the sintered bodies.